Patent GB-1439642-A (GB)
Title: Extruder screws and to extruder apparatus using such screws
IPC: B29C48/67
Filed: 1973-08-07
Abstract: 1439642 Extruder mixing screw KOEHRING CO 7 Aug 1973 [23 Aug 1972] 37412/73 Heading B5A [Also in Division B1] In a polymer plasticating extruder, such as in Fig. 1, a screw rotatably received in a cylindrical barrel has a feed section and a metering section, and the interthread channel of the metering section includes a wave form portion in which the depth of said channel varies cyclically along the channel length through minimum and maximum values for at least three cycles each extending over at least 180 degrees, said maximum value depth being less than the depth of the channel of the feed section. The zones of minimum and greater channel depths of said metering section serve respectively to enhance mixing and to obviate overheating of the polymer being advanced along the channel. Preferably, the minimum channel depth portions are in balanced relationship about the screw axis to balance side thrusts and avoid bending of the screw. The screw may be single- or multi-start on its wave form portions.  The typical extruder of Fig. 1 comprises a motor-driven screw 10 rotatably mounted in a barrel 8 having at the upstream end a feed opening 12 and at the downstream end a diecarrying swing gate 40 and a breaking plate 42 preventing passage through the die of impurities or solids. A transverse opening 32 of the barrel may be plugged for single-stage screw operation or, as shown, connected to a vacuum pump for two-stage screw operation in which the opening 32 communicates with a deep channel vent section of the screw where decompression and volatile release occurs. Associated with the barrel are heating and/or cooling means 22 to regulate melting, and cooling jackets 28, 30 receiving fluid from pumps 24, 26. The twostage screw 10 includes, successively, a first metering section having a wave-form portion 62, a vent section, and a second metering section of constant channel depth. The screw 44 of Figs. 2, 3 comprises, successively, a feed section 50 providing delivery rate and serving to compact and melt the polymer, a transition section 52, and a metering section 54 having upstream and downstream portions 56, 60 of constant channel depth and an intermediate single-start wave-form portion 58 of cyclically varying channel depth. The repeating wave form of the portion 58 may be symmetrical, e.g. a sine wave, or asymmetrical. As shown, each cycle extends over 540 degrees so that successive channel root peaks occur on opposite sides of the screw. Whilst not as illustrated, the screw is preferably "square pitch", i.e. with pitch equalling barrel bore diameter. In the single-start wave-form portion of Figs. 4, 5 (not shown), successive channel root peaks also occur on opposite sides of the screw, each cycle extending 180 degrees. In a two-start wave-form portion of a metering section (Fig. 6, 7, not shown), the channel of each start has successive root peaks at 1080 degrees so that they occur on the same side of the screw, and to balance the peaks along the screw axis, the alternate start channels are 180 degrees out of phase with each other. In Fig. 8 (not shown), a two-stage screw includes successively a constant depth channel metering section, a vent section, and a waveform channel section. In Fig. 9, a mixing barrier section 100 is disposed between wave-form and constant depth channel metering sections 98, 102 to supplement the action of the wave-form channel. The section 100 comprises longitudinal inlet and outlet channels 104, 108, and intervening set-back lands 106 over which the polymer passes. A metering section including a wave-form portion may or may not include a constant depth channel at one or both ends. The description includes specific screw dimensions, and refers to feeding polymer of powder or pellet form into the extruder and to extrusion of styrene sheet and vinyl pipe.